The optical waveguides or fibers used to transmit signals in optical telecommunication systems are characterized, in part, by the vector property of polarization mode dispersion (PMD). Polarization mode dispersion occurs as a result of birefringence in the fiber, which may be caused by physical asymmetry in the fiber construction itself, or by stress, strain, or other external forces imposed on the fiber. In addition, random polarization coupling can occur, giving rise to a time-varying statistical factor. Optical fibers display an anisotropy in the refractive index, which will vary as a function of position and time. Consequently, components of an optical signal that differ in polarization will propagate at different velocities, resulting in a differential group delay (DGD) between the components, and causing significant broadening of the optical pulses propagating along long lengths of fiber.
The PMD is fully characterized by a vector quantity {right arrow over (τ)}(ω) where the DGD is the magnitude of the vector |{right arrow over (τ)}(ω)|. As shown in FIG. 1, the DGD is generally designated as τ 10. Any state of polarization (SOP) can be resolved into directional components along two orthogonal principal states of polarization (PSP) 12, 14. The DGD or τ 10 then represents the separation in time between fast PSP 12 and slow PSP 14, after traversing a length of optical fiber 16. For each optical frequency or wavelength propagating in a fiber, there always exists two PSPs, such that the pulse spreading due to the first-order PMD vanishes if only one PSP is excited. The PMD is typically characterized in terms of an average DGD corresponding to different frequencies, and is independent, to first order, of wavelength, temperature, and external perturbations. In low mode coupled fiber, this measure of DGD averaged over a large range of optical frequencies is fairly constant over time, but in high mode coupled fibers, for example, in long fiber spans, the frequency-averaged DGD varies randomly in time, due to the combined effects of the variations in birefringence and random polarization mode coupling along the fiber length. This statistical variation in DGD lends itself to characterization of the DGD in terms of a statistical figure of merit, mean DGD.
Higher orders of the polarization mode dispersion also exhibit statistical properties. The effect of second-order polarization mode dispersion (SOPMD) 18 is shown in FIG. 1. The SOPMD is the first derivative of the PMD with respect to frequency, representing the change in the PMD as a function of frequency. The SOPMD, therefore, additionally characterizes the overall pulse spreading due to the frequency-dependence of the PMD and the spectral bandwidth of the injected optical pulse 19.
The polarization mode dispersion of a fiber is unlike most other sources of degradation in an optical telecommunication system, in its dependence on both time and frequency. Conventional methods for characterizing the full PMD vector over a frequency range, well known by those skilled in the art, include the Poincaré Sphere Analysis (PSA), the Jones Matrix Eigenvalue (JME), Müller Matrix Method (MMM), Fixed Analyzer and interferometric techniques. These methods provide a measure of mean DGD and root mean square (RMS) DGD, which is calculated from the set of frequency-dependent DGD values. It is then commonly assumed by those skilled in the art that the statistical DGD follows a Maxwellian distribution, so that a true mean DGD τ, determined by averaging the DGD values obtained for a number of fibers over a bandwidth B of optical frequencies, can be estimated by multiplying the measured RMS DGD
            〈              τ        2            〉        B  by a factor of
            8              3        ⁢        π              .
The fundamental problem in accurately evaluating a statistical limitation to an estimation of the mean DGD of a fiber, in order to find a more precise measurement of the mean DGD, was first recognized in a paper by N. Gisin, B. Gisin, J. P. Von der Weid, and R. Passy, entitled “How Accurately Can One Measure a Statistical Quantity Like Polarization-Mode Dispersion?” IEEE Photon. Tech. Lett., Vol. 12, pp. 1671-1673 (August 1996), which is incorporated herein by reference. The accuracy of mean DGD estimation does improve as the mean is taken over a larger spectral bandwidth (approaching the ideal theoretical case where B→∞). However, contrary to the statistical requirement that each of the measurements used to calculate an average be independent, the DGD at nearby wavelengths are not frequency independent. Gisin et al. demonstrated that this frequency dependence resulted in lower uncertainty in the mean DGD (around 9%) for larger PMD on the order of 1 picosecond (ps) e.g., as compared to a 28% uncertainty in mean DGD measurement when the PMD is smaller (on the order of 0.1 ps). The uncertainty in mean DGD measurement increases with decreasing source bandwidth. Gisin et al. demonstrated that the same level of uncertainty is intrinsic to all measurement techniques that average the DGD over wavelength.
The mathematical formalism was developed further by M. Shtaif and A. Mecozzi, “Study of the Frequency Autocorrelation of the Differential Group Delay in Fibers with Polarization Mode Dispersion,” IEEE Photon. Tech. Lett., Vol. 25, pp. 707-709 (May 2000), which is incorporated herein by reference. In measurements of the frequency autocorrelation of the DGD, the square DGD, and orientation of the PMD vector, Shtaif et al. showed that all corresponding correlation bandwidths are comparable. Shtaif et al. also showed that all statistical properties of the PMD characterizing the fiber under test are uniquely defined by the mean DGD.
Polarization mode dispersion (PMD) is recognized as a potentially limiting impairment for high-speed long-haul optical transmission. Moreover, precise measurement of the true mean differential group delay (DGD) of individual fiber links and whole fiber routes is important for accurate estimation of service outage probabilities. Since PMD varies with time, as well as with frequency, measurements of the mean frequency-averaged DGD of the same fiber taken at different times may differ from each other and from the true value of mean DGD for a given fiber. For DGD values in the usual range of interest, and within the optical bandwidths of commercially available equipment, the variance of DGD measurements is approximately inversely proportional to the optical bandwidth of the optical source used for the measurement. In other words, an accurate measurement of the mean DGD of low birefringence fiber is limited by the optical bandwidth of the source used for the measurement.
The need for precise PMD characterization will increase as the high-speed networks of the future employ very low PMD fibers. There exists a need, therefore, for more precise measurement of the mean DGD of individual fiber links and whole fiber routes than is presently provided by conventional methods.